Help – can’t get data from plugin table which I created in datebase

I use plugin create a table in database which name is “wp_ab_email”. I can use function insert content and it works very well.

But why I cannot use get_var(); retrieve data from it. Here is my code:
global $wpdb;
$temp=1;
$customer_email=$wpdb->get_var(“SELECT customer_email FROM $wpdb->ab_email WHERE order_num = $temp” );

{Here is the code from wp codex:
$name = $wpdb->get_var(“SELECT name FROM $wpdb->terms WHERE term_ID=4”);
}

As I see, it almost same, why I cannot get anything from it?